There are many exercises one can do to strengthen their arms. One of the best things to do is to go back to the basics.
One exercise you can do is to take yourself into tendu and place your arms in fourth position. Allow/feel your back and shoulder muscles fall down to relax and then extend again. Release and extend through your back, to your shoulders, to your elbows and then to your wrists.
After doing this eight times to the right and eight times to the left, you will begin to feel your back and shoulder muscles begin to engage.
You can also do this exercise in second position with your arms. The more time you take for yourself to work on this movement, the faster it will become second nature when put into choreography.